Hi,
There is a new release of mojoPortal coming very soon which includes changes to the forum thread count logic that should fix the problem. We got rid of the decrement and increment logic and made it use the sql count function whenever it updates the thread count. So after upgrading to the new version it will correct itself when a new thread or post is made.
In the meantime you could correct it by running a sql query to get the actual count.
Hope that helps,
Joe